cxgb4: Ignore MPS_TX_INT_CAUSE[Bubble] for T6

MPS_TX_INT_CAUSE[Bubble] is a normal condition for T6, hence
ignore this interrupt for T6.

Signed-off-by: Ganesh Goudar <ganeshgr@chelsio.com>
Signed-off-by: Casey Leedom <leedom@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>